BuildSupport: Require that the user provided a value for BUILDEM_REPO_DIR
lemon: Updated to lemon-1.2.4 (with minor patches)
Merge remote-tracking branch 'ilastik/master'

Conflicts:
	lemon.cmake
Deleted ilastik-specific CMakeLists.txt accidentally included in previous merge commit.
ExternalSource.cmake: external_git_repo(): Added support for an optional argument that allows the user to override the *_NAME and *_SRC_DIR variables set by the macro.
vigra.cmake: Provide custom package name to external_git_repo() macro.
ilastik.cmake: Override the ilastik_NAME and ilastik_SRC_DIR variables using the optional arg to external_git_repo()
ilastik-gui.cmake: Run the ilastik-gui test as a post-build command after the GUI target, not as a post-build step after the headless target.
vtk.cmake: Use $(MAKE) instead of make to allow parallel builds.